text: H-3 and H-6 in any 1,2-homodisubstituted benzene are related by a mirror plane of symmetry bisecting the 1,2 and 4,5 C-C bonds. They are therefore chemically equivalent (and magnetically equivalent by the chemical shift criterion) but, because they have different spatial and connectivity relations to H-4 (with 3-bond vs. 4-bond couplings of different strengths), they are magnetically inequivalent by the coupling criterion. The same is true with respect to their coupling relationships with H-5. Similarly, H-4 and H-5 are chemically equivalent but magnetically inequivalent owing to their different coupling relationships with H-3 (or H-6).
  A classic example showing highly complex splitting is that of 1,2-dichlorobenzene. The two signals are nearly mirror-symmetrical. In 1,2-diaminobenzene (ortho-phenylenediamine), the two signals have nearly the same chemical shift, so that the resultant signals form a complex multiplet.